1. Field of the Invention
This invention relates generally to silicon devices (including MEMS devices) and more specifically to a method for producing and testing a corrosion-resistant channel in a silicon device.
2. Description of the Background Art
A relatively recent development in the semiconductor industry is to use microelectromechanical systems (MEMS) in semiconductor and pharmaceutical manufacturing processes. MEMS devices are typically silicon chips that include miniaturized mechanical components, such as actuators, mirrors, levers, diaphragms, or sensors. MEMS devices may also include electronic circuitry.
When MEMS devices are employed in semiconductor and pharmaceutical manufacturing processes, they are exposed to the chemical and biochemical substances used in such processes. The part of the MEMS device exposed to fluids (i.e., gases or liquids) during operation is commonly referred to as the “wetted path.” The wetted path may be different from the primary flow path (i.e., the path along which the fluid is intended to travel) because fluids sometimes can enter into open spaces other than the primary flow path, referred to as the “dead volume”
The materials of the MEMS device that form the wetted path must be able to withstand corrosion or attack from fluids flowing through the device. In applications where corrosive fluids are present, the materials in the wetted path are critical, and compatibility of all the materials present is essential. In products requiring high purity, such as those used in the semiconductor or pharmaceutical industries, even a small amount of corrosion is unacceptable.
In many MEMS devices the wetted path is formed from a silicon channel, as MEMS devices are usually comprised at least in part by silicon wafers. The microvalve illustrated in FIGS. 1(a) and (b), in “off” and “on” states, respectively, is an example of a MEMS device with silicon in the wetted path. The valve is used to finely control the flow of fluids. The microvalve includes a heater plate 22, a diaphragm plate 28, and a channel plate 30.
The channel plate 30, which is formed from a silicon wafer, includes an input port 32 and an output port 34. The illustrated valve is a normally-open valve in that fluid entering input port 32 normally is able to travel freely through the valve 100 and out via output port 34, as depicted in FIG. 1(a). An example of a normally-closed valve is described in U.S. Pat. No. 6,149,123 (the “'123 patent”), the contents of which are incorporated by reference as if fully disclosed herein.
The diaphragm plate 28 includes a cavity 41 which holds a thermopneumatic liquid. The thermopneumatic liquid also extends up through channels 56 in the heater plate 22. When control circuitry (not shown) associated with the valve indicate the valve should close, the heater plate 22 warms the thermopneumatic liquid. The diaphragm plate 28, which is formed from a silicon wafer, includes a flexible diaphragm 44. When the thermopneumatic liquid is heated, it expands, causing the diaphragm 44 to bend and block input port 32. As illustrated in FIG. 1(b), when the input port 32 is blocked, the valve is closed and any fluid flow is severely restricted (e.g., less than 1 sccm).
The wetted path of valve 100 is cavity 43, the input and output ports 32, 34, and any exposed surfaces around the foregoing, all of which are formed from channel plate 30 and diaphragm plate 28. As these two plates 28, 30 are made of silicon wafers, the wetted path is a silicon channel.
A valve similar in operation to valve 100 is described in U.S. Pat. No. 4,996,646 (the “'646 patent”). Another example of a normally-open valve is described in U.S. Pat. No. 6,129,331 (the “'331 patent”). The contents of the '646 patent and the '331 patent are incorporated by reference as if fully disclosed herein.
As stated above, the fluids flowing through MEMS devices, such as the valve illustrated in FIGS. 1(a) and (b), must not corrode the device. For instance, if fluids were to sufficiently corrode the valve of FIGS. 1(a) and (b), the diaphragm 44, which is made up of a thin layer of silicon, would eventually break under operation. In addition, the cleanliness of the semiconductor or pharmaceutical process may be compromised by the products of the reaction of such fluids with the silicon. While silicon is non-reactive with most process gases and single constituent acids, it reacts with atomic fluorine, F, and other compounds which can spontaneously dissociate to atomic fluorine. A silicon atom, Si, will react with fluorine atoms to form SiF4, a volatile component which vaporizes off the surface, thereby corroding the silicon. Consequently, there is a need to protect the wetted path from fluorine.
Also, some liquid bases (e.g., pH >8) or mixed acids will corrode silicon, and, therefore, there is also a need to protect the wetted path from such fluids.
In semiconductor manufacturing processes that etch silicon with fluorine, a mask is often used to cover those portions of the wafer where etching is not desired. Such masks are made of materials which are unreactive or react very slowly with fluorine. Examples of such materials are SiO2, Si3N4, photoresist, or metal films of aluminum or nickel. However, these masks and the corresponding processes are used to selectively etch silicon and have not been employed to provide long-term protection of the wetted path of a MEMS device from corrosion by fluorine or other elements. In addition, such methods do not provide a means for identifying devices with inadequate coverage of the protective material.
Furthermore, such methods typically entail creating a protective metal film of aluminum or nickel by exposing aluminum or nickel layers to ClF3 gas or F2 gas, where the fluorine in these gases reacts with the metal to create a film, consisting of a nonvolatile fluorine compound, over the metal. The creation of the film provides a “passivating layer” on the aluminum or nickel. Materials, like aluminum and nickel, with which fluorine reacts to create a nonvolatile compound, are known to form these passivating layers. The problem with using ClF3 or F2 is that such gases are corrosive and highly toxic, rendering the passivation process dangerous, difficult, and expensive. For instance, exposure of silicon to ClF3 can produce extreme heat and may result in catastrophic failure of the MEMS device and associated equipment.
Applying materials, such as aluminum, nickel, or other protective layers, to the wetted path of a multilayer silicon MEMS devices presents an additional challenge. Some MEMS devices, such as valve 100, are comprised of two or more silicon wafers fusion bonded together. The fusion bonding creates hidden flow passages which are difficult to access using conventional deposition or electroplating techniques, and, thus such techniques are not suitable for multi-layer MEMS devices. Atomic layer deposition (“ALD”) processes can more easily reach such hidden passages, but known, true ALD techniques do not enable materials like aluminum to be deposited in layers thick enough to adequately protect the silicon.
The hidden passages in a MEMS device also present a challenge in ensuring complete protection of the wetted path. It is very important that potential defects in the protective film be screened out prior to use in a hostile environment.
Therefore, there is a need for a process for depositing, passivating, and testing a fluorine-resistant (and/or base or mixed acid resistant) material in the wetted path of a single or multilayer MEMS device that is reliable and complete and preferably employs less toxic and corrosive compounds than ClF3 or F2 to achieve the passivating layer.